1. Field of the Invention
This invention relates to low coherent reflectometers that use low coherent light beams for measuring reflectance and reflecting positions in measured optical circuits such as optical waveguides, optical modules, and the like.
2. Description of the Related Art
FIG. 8 shows a simplified configuration of a conventional low coherent reflectometer. Herein, reference numeral 100 designates a low coherent light source such as a light emitting diode (LED) that radiates low coherent light beams (simply, referred to as low coherent beams). One end of an optical fiber 101 is connected to an outgoing terminal of the low coherent light source 100. Reference numeral 102 designates an optical coupler having four ports, which are designated by reference numerals 102a to 102d respectively. The other end of the optical fiber 101 is connected to the port 102a of the optical coupler 102. In the optical coupler 102, low coherent beams incoming from the port 102a are subjected to branching in response to a prescribed intensity ratio (e.g., one-to-one ratio), so that branched beams are respectively output from the ports 102b and 102c. One end of an optical fiber 103 is connected to the port 102b of the optical fiber 102. The other end of the optical fiber 103 is connected to a measured optical circuit 104 which is a measured subject having a reflecting point therein.
One end of an optical fiber 105 is connected to the port 102c of the optical coupler 102. Reference numeral 106 designates a collimator lens whose focal point is set in advance and which is located at a terminal end 105a of the optical fiber 105. Reference numeral 107 designates a reflecting mirror for reflecting incoming beams that are transmitted thereto by way of the collimator lens 106. In addition, a stage (not shown) is provided to vary the distance between the collimator lens 106 and the reflecting mirror 107. One end of an optical fiber 108 is connected to the port 102d of the optical coupler 102, while the other end is connected to a received light signal processor 109. The received light signal processor 109 provides two light receiving elements (not shown) that respectively receive light beams entering from the optical fiber 108. The light receiving elements perform photoelectric conversion on the received light beams to produce electric signals. In addition, the light receiving elements also amplify differences between the electric signals.
Next, a description will be given with respect to the operations of the low coherent reflectometer shown in FIG. 8. First, low coherent beams generated by the low coherent light source 100 are subjected to branching by the optical coupler 102. The first of the branched beams are introduced into the measured optical circuit 104 as measurement beams by way of the optical fiber 103. Then, the measured optical circuit 104 produces reflected beams, which are transmitted back to the port 102b of the optical fiber 102 by way of the optical fiber 103.
The other of the branched beams output from the optical coupler 102 are introduced into the optical fiber 105 as local beams. Therefore, the local beams are output from the terminal end 105a of the optical fiber 105 and propagate towards the collimator lens 106. The collimator lens 106 converts them to parallel beams, which are then subjected to reflection of the reflecting mirror 107. The reflected beams are subjected to convergence by the collimator lens 106. The converged beams are introduced into the optical fiber 105 from its terminal end 105a Then, they are transmitted to the optical coupler 102 via the port 102c. 
In the optical coupler 102, the reflected measurement beams input from the port 102b and the reflected local beams input from the port 102c are combined. If the optical path for transmission of the measurement beams matches the optical path for transmission of the local beams, interference may occur in the optical coupler 102. Of the combined beams produced inside of the optical coupler 102, the beams output from the port 102d are subjected to photoelectric conversion and differential amplification by the light receiving elements, which are provided inside the received light signal processor 109.
It is possible to vary the spatial optical path length by moving the reflecting mirror 107 on the stage along the optical axis direction at a constant velocity. Therefore, it is possible to vary the optical path length for propagation of the local beams leaving from the port 102c of the optical coupler 102. The measurement beams travel from the port 102b of the optical coupler 102 to the measured optical circuit 104 via the optical fiber 103, so that the reflected measurement beams travel backwards by way of the optical fiber 103. Hence, the overall optical path length is established by the optical fiber 103 for transmission of the measurement beams. In addition, the local beams travel from the port 102c of the optical coupler 102 via the optical fiber 105 and also travel towards the reflecting mirror 107 via the collimator lens 106, so that the reflected local beams travel backwards by way of the collimator lens 106 and the optical fiber 105. Hence, the overall optical path length is established by the optical fiber 105, collimator lens 106, and reflecting mirror 107f or transmission and propagation of the local beams. When the overall optical path length of the measurement beams traveling between the port 102b of the optical coupler 102 and the measured optical circuit 104 is equal to the overall optical path length of the local beams traveling between the port 102c of the optical coupler 102, collimator lens 106 and reflecting mirror 107, interference occurs between these beams. Therefore, it is possible to measure the accurate position of the reflecting point in the measured optical circuit 104. Incidentally, details of the aforementioned technique are described in various papers such as Japanese Unexamined Patent Publication No. 2000-97856, for example.
In the aforementioned low coherent reflectometer, the measurement beams are transmitted through the optical fiber 103 only. That is, only a single optical fiber is used to form an optical path for transmitting the reflected measurement beams, which are produced by the measured optical circuit 104. As for the local beams, an overall optical path is composed of the optical fiber 105 and a spatial optical path which is formed across the terminal end 105a of the optical fiber 105, collimator lens 106, and reflecting mirror 107, wherein the spatial optical path has a refractive index of approximately xe2x80x981xe2x80x99.
As compared with the chromatic dispersions of the measurement beams and the reflected beams in the optical path formed by only the optical fiber 103, the chromatic dispersion of the local beams in the optical path decreases because of the existence of the spatial optical path, in which the local beams leaving from the terminal end 105a of the optical fiber 105 propagate towards the reflecting mirror 107 via the collimator lens 106 so that the reflected local beams propagate backwards to reach the terminal end 105a of the optical fiber 105. In other words, the spatial optical path causes a difference between the chromatic dispersions of the measurement beams and local beams. Such a difference adversely influences and deteriorates the spatial resolution in measurement of reflectance and the like.
In short, the reflecting point of the measured optical circuit 104 can be estimated by causing interference between the reflected measurement beams and the reflected local beams in the optical coupler 102, which is adjusted by varying the spatial optical path of the local beams in response to the movement of the reflecting mirror 107. As the spatial optical path becomes longer, the difference between the chromatic dispersion of the measurement beams and that of the local beams increases, which may result in deterioration of the spatial resolution.
It is an object of the invention to provide a low coherent reflectometer that can maintain a high spatial resolution in the measurement of reflectance even though the spatial optical path length for propagation of local beams is varied.
A low coherent reflectometer of this invention uses low coherent beams for the measurement of reflectance with respect to a measured optical circuit including a reflecting point. According to the first aspect of the invention, the low coherent beams are branched by an optical coupler to produce measurement beams and local beams. The measurement beams are introduced into a first optical path, which includes a dispersion shifted fiber, towards the measured optical circuit, while the local beams are introduced into a second optical path, which includes a spatial optical path, terminated by a reflecting mirror. Reflected measurement beams and reflected local beams are combined together to produce combined beams, which are subjected to processing and analysis. The spatial resolution is noticeably improved even though the spatial optical path length is varied because the length of the dispersion shifted fiber is determined so as to substantially match the length of the spatial optical path for propagation of the local beams towards the reflecting mirror.
According to the second aspect of the invention, an optical bandpass filter is provided to restrict the wavelengths of the low coherent beams within a prescribed range of wavelengths. That is, the optical bandpass filter has specific transmission characteristics to adjust the full width at half maximum (FHM) in the spectrum of the low coherent beams, thus minimizing effects (or influences) on spatial resolutions due to chromatic dispersions. The beams transmitted through the optical bandpass filter are branched to produce measurement beams and local beams. The measurement beams are directly transmitted towards the measured optical circuit without the intervention of the dispersion shifted fiber. The local beams are introduced into the second optical path, which includes the spatial optical path. Incidentally, the spatial optical path is formed by a collimator lens and a reflecting mirror, which are spaced apart by a prescribed distance.